1. Field of the Invention
The present invention relates to a system for detecting an alarm (hereinafter, an alarm system) for detecting emergencies in a predetermined guard area of a building or house, such as unauthorized intrusions, cutting of wires or short-circuit by intruders, and nonsecured doors or windows, using alarm switch units installed to the doors or windows.
2. Description of the Prior Art
A conventional alarm system disclosed in Japanese Examined Patent Publication (Kokoku) No. 57-16395, as shown in FIGS. 1 and 2, comprises permanent magnets 2 installed to the upper portion of doors 1; alarm switch units S.sub.11 to S.sub.1n having lead switches installed to the upper portion of door frames 3 corresponding to the magnets 2, for detecting unauthorized intrusions, nonsecured doors, cutting of wires, or short-circuits by intruders in response to removal of the magnetic flux of the magnets 2; and an alarm device AL including a power source E, buzzer BZ, and lamp L, and a detecting circuit DET.
The alarm switch units S.sub.11 to S.sub.1n are connected in a circuit. Each consists of a first contact 12 which opens at emergencies, a second contact 13, and a main contact 11. A resistor R is connected between the first contact 12 and the second contact 13 of the alarm switch S.sub.1n. A constant current flows in the circuit under ordinary conditions. When the wire (W) is cut at any point in the circuit, or a door is opened by an intruder, the circuit changes an current value. This change is detected by the detecting circuit DET, whereby the buzzer BZ or lamp L indicates the emergency.
However, in this alarm circuit, a constant current must always flow in the circuit, which entails high power consumption.